Problem Overview

The course covers basic algorithmic techniques and ideas for computational problems arising frequently in practical applications: sorting and searching, divide and conquer, greedy algorithms, dynamic programming. We will learn a lot of theory: how to sort data and how it helps for searching; how to break a large problem into pieces and solve them recursively; when it makes sense to proceed greedily; how dynamic programming is used in genomic studies. You will practice solving computational problems, designing new algorithms, and implementing solutions efficiently (so that they run in less than a second).
Do you have technical problems? Write to us: coursera@hse.ru

教學方

Alexander S. Kulikov

Visiting Professor

Michael Levin

Lecturer

Neil Rhodes

Adjunct Faculty

Pavel Pevzner

Professor

Daniel M Kane

Assistant Professor

腳本

Hello, and welcome to the next lesson in the dynamic programming module. In this lesson, we will be applying the dynamic programming technique for solving a wide range of problems where your goal is to find an optimal order of something. We will illustrate this technique by solving the so-called placing parentheses problem. In this problem, your input is an arithmetic expression consisting of numbers or digits and arithmetic operations, and your goal is to find an order of applying these arithmetic operations that maximizes the radian. You specify this order by placing parentheses, and that's why the problem is called placing parentheses. As usual we start with problem overview. Consider the following toy arithmetic expression. 1 + 2- 3 x 4- 5. In this case we have five digits and four arithmetic operations. And we would like to find an order of applying these four arithmetic operations to maximize the value of this expression. So when the order of operation is fixed, you do the following. You take the first operation. You take two adjusting digits, and you apply these operations. For example, if the operation is multiplication, in this case, so then two digits are 3 and 4. So you multiply 3 and 4, you get 12, and you just replace 3 times 4 by 12. You then take the next operation, apply it also, and replace two numbers and the arithmetic sign by this result, until you proceed in a similar fashion. In the end here, you get a single number. And your goal is to find an order that guarantees that this number is as large as possible. You can specify an order just by placing a set of parentheses in your expression. For example, if you would like to apply all your four operations just from left to right, you place the parentheses as follows. In this particular case, we compute the results as follows. So we first compute 1 + 2, this is 3. We then subtract 3 from the results. This gives us 0. We then multiply the result by 4. This is still 0. And finally, we subtract 5. So this gives us -5. And this is actually non-optimal, because for example, there is a better order. In this case, we first multiply 3 and 4, this gives us 12. We then subtract 5, this gives us 7. Then we go to compute the sum of 1 and 2, this gives us 3. So when the final operation is subtraction, we subtract 7 from 3. This gives us -4. So in this case the order of applying operations was the following. So we first compute the product of 3 and 4, so this is the first operation. We then subtract 5. This is the second operation. We then compute the result of 1 + 2. So this plus is the third operation, and this minus is the fourth operation, the last one. It is not difficult to see that the optimal value in this case is equal to 6. And it can be obtained as follows. You first subtract 5 from 4. This gives you -1. You then multiply it by 3, and you get -3. You then compute the sum of the first two digits. This is 1 + 2, and that is equal to 3. Finally you subtract -3 from 3. This is the same as 3 + 3, it is equal to 6. Well, you might find the result as follows, you just go through all possible orders. Let's see how many different orders are there. Well, there are four arithmetic operations in this case, so you can choose any of the four possible arithmetic operations to be the first one. You can choose any of these three remaining operations to be the second one, and you can select any of the two remaining operations to be the third one. And the last one is unique, it is the only remaining operations. So, in total, there are 4 by 3 by 2 by 1 different orders. This is equal to 24, and you can just enumerate all of them, write them down, compute an answer for each of these orderings and select the maximal value. However, our method of going through all possible orderings does not scale well. And this is why. Consider the toy example shown on the slide. In this case we have six digits and five arithmetic operations. This example will require us to go through all possible 120 orderings. So just because there are five iterations, so any of five of them can be the first one, any of the remaining four of them can be the second one, and so on. So this is 5 by 4 by 3 by 2 by 1, which is equal to 120. This is already not so easy to do this by hand. I mean, to go through all possible such orderings. Well, this is not easy, but we can teach a computer to do this, right? So we can implement an algorithm that goes through all possible orderings. However, in general, this algorithm will perform roughly n factorial steps, where n is the number of arithmetic operations, for exactly the same reason. If you have n arithmetic operations, then any of them can be the first one. Any of the remaining n minus 1 operations can be the second one, and so on. So this is n times n minus 1, times n minus 2, and so on. This is equal n factorial, and n factorial is an extremely fastly growing function. For example, 20 factorial already equals roughly 2 times 10 to the 18. This means that if you implement such an algorithm, it will not be able to compute the maximum value of an expression consisting of just 20 digits in a reasonable time, even in one year, not to say about one second. Which means, as usual, that we need another algorithm, as you might well have guessed. We will use dynamic programming to find, to design a more efficient algorithm. In the meantime, you might want to check your intuition by trying a few possible orderings to perform in this small expression, and by using our in video quiz.